Somatosensory determinants of lordosis in female rats: behavioral definition of the estrogen effect.

By coating the ventral surface of male rats with a dye, regions of contact between male and female during male mounting were recorded precisely on the female's hair and skin. The male rat touches the female's flanks, rump, tailbase, perineum, and perivaginal surfaces during the female's initiation and maintenance of lordosis. Film analyses showed that the male's paws and pelvic thrusting stimulate the female's skin with dominant frequencies between 10 and 20 per second. Somatosensory stimuli were applied by the experimenter to the female skin locations contacted by the male. Deflection of hair on the flanks or perineum alone did not cause lordosis. Light stimulation simultaneously on flanks and perineum caused lordosis only in some females given high estrogen dosages supplemented by progesterone. When flank stimuli were followed by pressure on the rump, tailbase, and perineum, lordosis was triggered reliably in hormone-treated females. Here the estrogen-dependence of the reflex was shown, and progesterone synergized with the estrogen effect. Among lordosis components, rump and head elevations in response to pressure stimuli on the rump, tailbase, and perineum appear to be hormone-sensitive. These results help to define the minimal cutaneous sensory requirement for lordosis. In turn, the estrogen effect on lordosis may be defined behaviorally as increased responsiveness to pressure on rump, tailbase, and perineal skin, after flank stimulation. These results illustrate how estrogen, progesterone, and somatosensory stimuli interact in causing lordosis, increases in the strength of one factor compensating for decreases in another.